High voltage load resistor array

ABSTRACT

A high voltage resistor comprising an array of a plurality of parallel electrically connected resistor elements each containing a resistive solution, attached at each end thereof to an end plate, and about the circumference of each of the end plates, a corona reduction ring. Each of the resistor elements comprises an insulating tube having an electrode inserted into each end thereof and held in position by one or more hose clamps about the outer periphery of the insulating tube. According to a preferred embodiment, the electrode is fabricated from stainless steel and has a mushroom shape at one end, that inserted into the tube, and a flat end for engagement with the end plates that provides connection of the resistor array and with a load.

The United States of America may have certain rights to this inventionunder Management and Operating Contract No. DE-AC05-84ER 40150 from theDepartment of Energy.

FIELD OF THE INVENTION

The present invention relates to high voltage resistors and moreparticularly to such a high voltage resistor that is capable of handlingupwards of 600,000 volts DC at a current of 2 amps or more withoutarcing or surface breakdown.

BACKGROUND OF THE INVENTION

In certain leading edge technological applications such as the operationof free electron lasers and the like, there exists the need to be ableto safely handle very high voltages, on the order of above 500,000volts, in, for example, power supplies and the like. In suchapplications, load resistors capable of handling such voltages are anecessary requirement. In such applications, the presence of electricalcurrent on the order of 2 amps or higher is also quite possible.

Currently there are only a few commercially available options forresistors capable of handling such loads. Among these are solid carbonresistors and high resistance metal alloy load banks. While solid carbonresistors are capable of handling such loads, they are very expensive toconstruct, require long lead times to obtain, are not adjustable toohmic values other than those for which they were designed and built, i.e. not flexible, and if high voltage flash-over or arcing occurssignificant damage will be inflicted and the costly resistor will haveto be replaced because repair is not normally an option. Metal alloybanks demonstrate similar shortcomings in that they are even moreexpensive to construct, require long lead times to obtain, are notadjustable to ohmic values other than those for which they were designedand built and furthermore, are usually designed to handle larger currentloads at much lower operating voltages than those encountered in, forexample, the operation of free electron lasers as just described. Thus,while such prior art devices may meet the needs of certain applications,their high cost, relative inflexibility in terms of modification forload variation, and their relative inability to be readily repaired makethem inappropriate for use in high voltage applications that may requirevoltage or amperage handling variation and could result in damage to theresistor.

Thus, there exists a need for a relatively inexpensive resistor systemcapable of handling very high voltages on the order of hundreds ofthousands of volts, which resistor system possesses the ability to bereadily modified to change is voltage/amperage handling characteristicsand which can be readily repaired in the case of high voltage flash-overor arcing event.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a highvoltage resistor system that is relatively inexpensive to produce andwhich can be readily modified to alter its voltage/amperage handlingcapabilities.

It is another object of the present invention to provide a high voltageresistor system that, in the case of voltage flash-over or arcing orother damaging failure can be readily repaired at low cost.

SUMMARY OF THE INVENTION

According to the present invention there is provided a high voltageresistor comprising an array of a plurality of parallel electricallyconnected resistor elements each containing a resistive solution,attached at each end thereof to an end plate, and about thecircumference of each of the end plates, a corona reduction ring. Eachof the resistor elements comprises an insulating tube having anelectrode inserted into each end thereof and held in position by one ormore hose clamps about the outer periphery of the insulating tube.According to a preferred embodiment, the electrode is fabricated fromstainless steel and has a mushroom shape at one end, that inserted intothe tube, and a flat end for engagement with the end plates thatprovides connection of the resistor array and with a load.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the resistor array of the presentinvention.

FIG. 2 is a cross sectional view of one embodiment of the resistor arrayof the present invention.

FIG. 3 is an exploded view of one embodiment of a liquid containingresistor element in accordance with the present invention.

FIG. 4 is a partially phantom view of the electrode portion of theresistor array of the present invention.

DETAILED DESCRIPTION

Referring now to the accompanying figures, the resistor array 10 of thepresent invention comprises an array of resistor elements 12 which arrayhas first and second ends 14 and 16. Located about ends 14 and 16 arecorona rings 18 that serve to protect resistor array 10 from damage byreducing the possibility of arcing or voltage flash-over. Each ofresistor elements 12 is contactingly attached to end plates 13 and 15 asdescribed below. Bottom end plate 15 is in turn preferably attached to acarriage 17 for ease of movement of resistor array 10 from location tolocation.

Corona rings 18 serve to minimize the possibility that any sharp edgeson end plates 13 and 15 (described below) would emit corona in the caseof a high voltage application resulting in arcing or voltage flash overand preferably form an integral part of end plates 13 and 15 (describedbelow). Corona rings 18 must be of a size adequate to provide a requiredsafety margin in the case of any potential arcing or voltage flash over.In the case of a resistor array 10 designed to handle on the order of600 KV, corona rings on the order of 3 inches in diameter are generallyadequate while end plates 13 and 15 are of a diameter of about 14inches. In addition to protecting against corona discharge, corona rings18 help to grad the electric fields evenly between end plates 13 and 15thus reducing the chances of a high voltage arc over from high voltageinput cable 46 to bottom end plate 15 or the ground plane end ofresistor array 10. In the situation where resistor array 10 was exposedto an applied voltage of one Megavolt or more, corona rings 18 wouldhave to be enlarged to on the order of 6 inches in diameter tofacilitate electric field grading between end plates 13 and 15 and toprovide the required arcing protection. While corona rings 18 arediepicted in the accompanying figures as integral portions of end plates13 and 15, it will be readily understood that corona rings 18 could alsobe welded or otherwise attached to end plates 13 and 15 so as to renderthem integral parts of end plates 13 and 15.

As shown in FIG. 3, each of resistor elements 12 comprises an insulatingtube 20 containing a resistive fluid as described below and having anelectrode 22 inserted into opposing ends 24 and 26 thereof. Electrode 22preferably has a preferably mushroom shaped end 28 for ease of insertioninto end 26 of insulating tube 20 and a flat end 30 for contacting anelectrode plate for electrical connection with the balance of theresistor system as described below. As will be apparent to the skilledartisan, mushroom shaped end 28 could be of any suitable shape thatallows for insertion of end 28 into insulating tube 20. According to theembodiment depicted in FIGS. 3 and 4, electrodes 22 are provided withperipheral ridges 30 that provide for a tight interference fit whenelectrode 22 is inserted into ends 24 and 26 and also providedepressions 32 into which hose clamps 34 and 36 can rest as hose clamps34 and 36 are tightened in the conventional fashion, and insulated tube20 deforms under the peripheral pressure of hose clamps 34 and 36.Clearly, specific alternative structures for electrodes 22 can beprovided without departing from the spirit and scope of the invention.While insulating tubes 20 can be fabricated from a wide variety ofmaterials, it is preferred that they be of Tygon® or some similar highlyinsulating and heat resistant material.

As best seen in FIG. 4, electrode 22 in addition to the elements thereofpreviously described also incorporates apertures 38, 39 and 40 thatreceive bolts 42 that serve to attach resistor electrodes 22 andconsequently resistive fluid containing resistor elements 12 to endplates 13 and 15. The insertion and tightening of bolts 42 throughapertures in end plates 13 and 15 into apertures 38 and 40 provide thatthe entire resistor array is securely and electrically connectivelyjoined together as a unitary structure comprising resistor elements 12and end plates 13 and 15 that, in turn, are attached to corona rings 18.While three bolt apertures 38, 39 and 40 are depicted in FIG. 4, it willbe readily apparent that two or even a single bolt aperture and bolt 42can be provided so long as electrodes 22 as sufficiently tightlyattached to end plates 13 and 15 as to provide electrical contactbetween electrodes 22 and end plates 13 and 15. High voltage isolationsupport rods 21 are preferably used to provide additional structuralsupport to resistor array 10 as insulated tubes 20 may not providesufficient structural strength to support the entire structure. Supportrods 21 are, of course, joined to end plates 13 and 15 using suitableinsulating connectors in a fashion well known to those skilled in theart of constructing such devices. A suitable high voltage power supply44 is connected to resistor array via high voltage cable 46 and groundreturn cable 48. The foregoing comprises a description of the essentialelements of resistor array 10.

In many applications, it may be necessary that resistor array 10 bemoved from location to location. In such an instance it may be desirableto equip resistor array 10 with a suitable carriage arrangement and thatfeature is now described. Carriage 17 comprises a platform 52 equippedwith casters 54 and carriage 17 is attached to resistor array 10 throughthe mechanism of a plurality of legs 56 that are attached to andinsulated from bottom end plate 15, for example through the use ofinsulating structures 58, or other similarly insulated structures wellknown in the art. When thus equipped with a carriage, resistor array 10can be moved with relatively little effort to whatever location mayrequire its use.

An obviously critical element of the resistor array 10 of the presentinvention is, of course, the resistive fluid contained within resistorelements 12. According to a preferred embodiment of the presentinvention, resistive fluid contained within resistor elements 12comprises an aqueous solution of copper sulfate. As is apparent, a widevariety of other resistive fluids could be used as the resistive fluidin resistor array 10. The value of resistor array 10 is determined byboth the number of resistor elements and the concentration of coppersulfate or other appropriate resistive fluid in a polar solvent such aswater. In the case of a copper sulfate or similar resistive solution,the maximum resistive value (˜a few meg-ohms) will be obtained when puredistilled water is used. The lowest resistive values (˜a few hundredohms) will be obtained by having the maximum amount of copper sulfate orother solute that can be dissolved in water. Although there are formulaefor determining the end value of a resistor array constructed given adiscreet insulating tube length, tubing cross section, number ofresistive elements and the resistivity of the solution (such formulaebeing well known to those skilled in the art of designing fluidresistors) such formulae depend to some extent on the quality of thedistilled water or other solvent used and the purity of the solute, inthe preferred case copper sulfate or other suitable solute. Throughexperience, it has been found that simply preparing an approximatelyappropriate resistive solution, measuring its resistivity with a meterand then adjusting the concentration of the solute in the solutionupwards (to decrease resistivity) or downwards (to increase resistivity)works extremely well for determining the proper solution given thematerials being used and the design of resistor 10, i.e. the number ofresistor elements used. A high voltage Megger insulation test meter hasbeen found entirely adequate to measure the resistance of such fluidsduring the fabrication process. Since the design of the resistor arrayas described above comprises several resistors in parallel to distributethe current load, the resistive value of all resistors should be withina maximum range of about 20% of each other's value (a limit of 10% aboveor below the desired value). This prevents one resistor element fromcarrying too large a share of the current and overheating. A resistorarray comprising 12 resistor elements about 33 inches long has beenfound to be entirely adequate for purposes of a 600,000 volt load at lowamperage as described above.

Fabrication of the resistor array 10 of the present invention isaccomplished by first inserting one of the electrodes 22 into the firstend of one of the insulating tubes 20, applying hose clamps 34 and 36about the periphery of insulating tube 20 such that hose clamps 34 and36 depress a portion of insulating tubing 20 into recesses 32 to closethe first end of the insulating tube 20. Secondly, filling the thusformed closed insulating tube 20 with a suitable resistive solutionprepared as described above and repeating the insulating tube closingoperation by insertion of an electrode 22 into the second end of thetubing and applying hose clamps 34 and 36 as just described. A pluralityof resistor elements 12 adequate in number for the particularapplication being designed is prepared and the resistor elementsconnected to end plates 13 and 15 by the insertion of bolts 42 intothrough apertures in end plates 13 and 15 and into apertures 38 and 40in electrodes 22. High voltage isolation support rods 21 are similarlybolted into place between end plates 13 and 15. If desired, the thusassembled resistor array can then be mounted to a carriage as describedhereinabove.

There has thus been described a high voltage resistor that is readilymodifiable to meet a wide variety of resistor applications and isreadily repairable in case of physical damage due to electrical overloador failure or simple mishandling.

While high voltage resistor 10 of the present invention has beendescribed and shown in the Figures as being circular as this isobviously the most compact and efficient design for ease movement andelectrical protection, it will be apparent to the skilled artisan thatother somewhat less efficient designs ranging from rectangular (harderto protect from arcing) to oval may also be constructed withoutdeparting from the spirit and scope of the invention. What ever thedesign, it is very important that no sharp edges, burrs or the like beresent on the surface of any of the conductive elements as such defectscan result in unwanted and even dangerous sites for the occurrence ofarcing. Similarly, all surfaces should be clean and free of potentiallyconductive oils, greases and the like to prevent unwanted arcing duringuse.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1. A high voltage resistor comprising: a) an array of a pluralityresistor elements having first and second ends and each of said resistorelements comprising: i) an insulating tube containing a resistive fluidand having an electrode inserted into each end thereof; b) end plates ateach of said first and second ends attached to said resistor elements;and c) about the circumference of each of said end plates, a coronareduction ring.
 2. The high voltage resistor of claim 1 furtherincluding high voltage isolation and support rods between said endplates.
 3. The high voltage resistor of claim 1 wherein said electrodesare of stainless steel.
 4. The high voltage resistor of claim 1 whereinsaid electrodes have first and second ends, said first end is mushroomshaped for ease of insertion into said insulating tubes and said secondend is flat.
 5. The high voltage resistor of claim 1 wherein saidelectrodes include at least two peripheral ridges that define a recesstherebetween, said insulated tube has a periphery and said electrodesare retained in said insulated tube by means of at least one hose clampabout said periphery and said electrode in the region of said recess. 6.The high voltage resistor of claim 1 wherein said electrodes each have aflat end that engages one of said end plates, said end plates includeend plate apertures, said flat ends include electrode apertures andbolts inserted through said end plate apertures into said electrodeapertures such that said bolts attach said resistor elements to said endplates.
 7. The high voltage resistor of claim 1 wherein said resistivesolution comprises a solution of copper sulfate.
 8. A high voltageresistor comprising: a) an array of a plurality resistor elements havingfirst and second ends and each of said resistor elements comprising: i)an insulating tube having first and second ends and containing aresistive fluid; ii) stainless steel electrodes having a mushroom shapedfirst ends inserted into each of said first and second insulating tubeends, a flat end, bolt apertures in said flat end and at least twoperipheral ridges defining a recess therebetween; iii) at least one hoseclamp about each of said first and second ends of said insulating tubeand each of said stainless steel electrodes retaining said electrodes insaid first and second ends of said insulating tube; iv) end plates ateach of said first and second ends having end plate apertures thereinattached to said resistor elements by bolts that penetrate said endplateapertures and fasten in said electrode apertures; and v)about thecircumference of each of said end plates, a corona reduction ring. 9.The high voltage resistor of claim 1 further including a carriage towhich said high voltage resistor is attached.
 10. The high voltageresistor of claim 8 further including a carriage to which said highvoltage resistor is attached.